High-strength aluminum alloy and manufacturing method thereof

ABSTRACT

An aluminum alloy suitable for anodizing contains, in mass percent, Zn: 5.0% or more and 7.0% or less, Mg: more than 2.2% and 3.0% or less, Cu: 0.01% or more and 0.10% or less, Zr: 0.10% or less, Cr: 0.02% or less, Fe: 0.30% or less, Si: 0.30% or less, Mn: 0.02% or less, and Ti: 0.001% or more and 0.05% or less, the remainder being composed of Al and unavoidable impurities. The aluminum alloy has a Zn/Mg ratio of 1.7 or more and 3.1 or less, a proof stress of 350 MPa or more and a metallographic structure composed of a recrystallized structure.

TECHNICAL FIELD

The present invention relates to a high-strength aluminum alloy that canbe used in parts where at least both appearance characteristics andstress-corrosion-cracking resistance are considered to be important.

BACKGROUND ART

Aluminum alloys are being increasingly employed as materials for use insports equipment, transportation equipment, machine parts, and otherapplications wherein at least strength properties and appearancecharacteristics are considered to be important. Because durability isrequired for these applications, it is desirable to use high-strengthaluminum alloys having a proof stress of 350 MPa or more. For example,the aluminum-alloy extruded material described in Patent Document 1 hasbeen proposed as an aluminum alloy for use in applications wherein bothstrength properties and appearance characteristics are considered to beimportant.

PRIOR ART LITERATURE Patent Documents

Patent Document 1

Japanese Laid-open Patent Publication 2012-246555

SUMMARY OF THE INVENTION Problems to be Solved by the Invention

In the 7000-series aluminum alloy described in Patent Document 1, thereis a problem in that, when a T6 process is performed with an agingtreatment, stress corrosion cracking tends to occur. In addition, thereis a problem in that, if an overaging treatment is performed as acorrective measure therefor, then strength decreases even thoughstress-corrosion-cracking resistance can be improved.

Thus, even though the previously-existing 7000-series aluminum alloydescribed in, for example, Patent Document 1 has high proof stress, itcannot be said that Patent Document 1 takes corrective measures withregard to the stress-corrosion-cracking characteristic. Consequently,this alloy is not suited to applications in which the alloy is used overan extended period of time in a state wherein the alloy is continuouslysubject to stress in a corrosive environment.

The present invention was conceived against this background, and anobject of the present invention is to provide a high-strength aluminumalloy that excels in surface quality and stress-corrosion-crackingresistance after an anodization treatment, and a manufacturing methodtherefor.

Means for Solving the Problems

In a first aspect of the invention, a high-strength aluminum alloy to besubjected to an anodization treatment comprises:

-   -   a chemical composition containing, in mass %, Zn: 5.0% or more        and 7.0% or less, Mg: more than 2.2% and 3.0% or less, Cu: 0.01%        or more and 0.10% or less, Zr: 0.10% or less, Cr: 0.02% or less,        Fe: 0.30% or less, Si: 0.30% or less, Mn: 0.02% or less, and Ti:        0.001% or more and 0.05% or less, the remainder being composed        of Al and unavoidable impurities and a Zn/Mg ratio being 1.7 or        more and 3.1 or less;        wherein,    -   the proof stress is 350 MPa or more; and    -   the metallographic structure is composed of a recrystallized        structure.

In another aspect of the present invention, a method of manufacturingthe high-strength aluminum alloy comprises the steps of:

-   -   preparing an ingot having a chemical composition containing, in        mass %, Zn: 5.0% or more and 7.0% or less, Mg: more than 2.2%        and 3.0% or less, Cu: 0.01% or more and 0.10% or less, Zr: 0.10%        or less, Cr: 0.02% or less, Fe: 0.30% or less, Si: 0.30% or        less, Mn: 0.02% or less, and Ti: 0.001% or more and 0.05% or        less, the remainder being composed of Al and unavoidable        impurities and a Zn/Mg ratio being 1.7 or more and 3.1 or less;    -   performing a homogenization treatment that heats the ingot at a        temperature above 540° C. and 580° C. or lower for 1-24 h;    -   hot working the ingot, in a state wherein the temperature of the        ingot at the start of the working has been set to 440-560° C.,        thereby making it a wrought material;    -   performing a quenching treatment that cools by controlling,        after cooling has started while the temperature of the wrought        material is 400° C. or higher, an average cooling rate while the        temperature of the wrought material is in the range of 400° C.        to 150° C. such that it is 1° C./s or more and 300° C./s or        less;    -   cooling the temperature of the wrought material to room        temperature by the quenching treatment or by cooling thereafter;        and    -   subsequently performing an artificial-aging treatment on the        wrought material.

Effects of the Invention

The above-mentioned high-strength aluminum alloy has the above-mentionedspecific chemical composition, the proof stress being 350 MPa or moreand the metallographic structure being composed of a recrystallizedstructure. Thereby, the above-mentioned high-strength aluminum alloy ishigh strength, excels in stress-corrosion-cracking resistance, andexcels in surface quality after the anodization treatment, and can besuitably used in a part in which the strength properties, the appearancecharacteristics, and stress-corrosion-cracking resistance are consideredto be important.

That is, the above-mentioned high-strength aluminum alloy has theabove-mentioned specific chemical composition, and thereby an excellentstress-corrosion-cracking resistance characteristic can be ensured;thereby, the high-strength aluminum alloy can exhibit excellentdurability even when used in a corrosive environment.

In addition, the above-mentioned high-strength aluminum alloy has aproof stress equal to or greater than that of the above-mentioned,previously-existing 7000-series aluminum alloy, that is, a proof stressof 350 MPa or more. Consequently, it is possible to relatively easilymeet the requirements for strength, such as ensuring strength propertiesthat can support, for example, a reduction in wall thickness in order toreduce weight.

In addition, because the above-mentioned high-strength aluminum alloyhas the above-mentioned specific chemical composition and ametallographic structure composed of a recrystallized structure, theformation of streak patterns, caused by fibrous structures after theanodization treatment, and the like can be inhibited, making it possibleto obtain a satisfactory surface quality.

Next, in the above-mentioned high-strength aluminum alloy materialmanufacturing method, the above-mentioned high-strength aluminum alloyis manufactured based on the above-mentioned specific treatmenttemperatures, treatment times, and treatment procedures. Consequently,the above-mentioned excellent high-strength aluminum alloy can be easilyobtained.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a photograph, as a substitute for a drawing, that shows themetallographic structure of sample 4 according to Working Example 1.

FIG. 2 is a photograph, as a substitute for a drawing, that shows themetallographic structure of sample A19 according to Working Example 1.

MODE(S) FOR CARRYING OUT THE INVENTION

The above-mentioned high-strength aluminum alloy has a chemicalcomposition that contains, in mass %, Zn: 5.0% or more and 7.0% or less,Mg: more than 2.2% and 3.0% or less, Cu: 0.01% or more and 0.10% orless, Zr: 0.10% or less, Cr: 0.02% or less, Fe: 0.30% or less, Si: 0.30%or less, Mn: 0.02% or less, and Ti: 0.001% or more and 0.05% or less,wherein the remainder is composed of Al and unavoidable impurities, andthe Zn/Mg ratio is 1.7 or more and 3.1 or less. First, the reasons forthe range limits of the content of each element will be explained.

Zn: 5.0% or More and 7.0% or Less

Zn is an element that coexists with Mg in the aluminum alloy and therebyprecipitates the η′ phase. By containing Zn together with Mg, it ispossible to obtain an increase in strength due to enhancedprecipitation. If the Zn content is 5.0% or less, then the η′ phaseprecipitated amount becomes small, and consequently the strengthimproving effect is lowered. Consequently, the Zn content is preferablygreater than 5.0%, and more preferably 5.2% or more. On the other hand,if the Zn content exceeds 7.0%, then stress-corrosion-crackingresistance deteriorates. Consequently, the Zn content is preferably 7.0%or less and more preferably 6.8% or less.

Mg: More than 2.2% and 3.0% or Less

Mg is an element that coexists with Zn in the aluminum alloy and therebyprecipitates the η′ phase. By containing Mg together with Zn, it ispossible to obtain an increase in strength due to the enhancement ofprecipitation. If the Mg content is 2.2% or less, then the η′ phaseprecipitated amount becomes small, and consequently the strengthimproving effect is lowered. On the other hand, if the Mg contentexceeds 3.0%, then hot workability decreases, productivity decreases,and stress-corrosion-cracking resistance deteriorates.

Zn/Mg Ratio: 1.7 or More and 3.1 or Less

The contents of Zn and Mg are selected such that they are each withinthe limit ranges described above and such that the value of theabove-mentioned Zn quantity/Mg quantity ratio is definitely in the rangeof 1.7-3.1. If the Zn/Mg ratio is less than 1.7, then strength tends tobecome low; on the other hand, if it exceeds 3.1, thenstress-corrosion-cracking resistance deteriorates. Furthermore, theZn/Mg ratio means the Zn content (mass %)/Mg content (mass %) value.

Cu: 0.01% or More and 0.10% or Less

If a recycled material is used as the raw material of theabove-mentioned high-strength aluminum alloy material, then Cu might beintermixed therein. If the Cu content exceeds 0.10%, then it leads to areduction in surface quality, such as a decrease in luster afterchemical polishing, a change in the color tone to yellow caused by theanodization treatment, and the like; and if the Cu content is less than0.01%, then stress-corrosion-cracking resistance deteriorates. Such adeterioration in stress-corrosion-cracking resistance and surfacequality can be avoided by restricting the Cu content to 0.01% or moreand 0.10% or less.

Zr: 0.10% or Less

If the Zr content exceeds 0.10%, then the formation of a recrystallizedstructure is inhibited and, instead, fibrous structures tend to beformed. If the above-mentioned fibrous structures are present, thenafter the anodization treatment is performed, streak patterns caused bythe fibrous structures tend to appear on the surface, and consequentlythere is a risk that surface quality will decrease. Consequently, the Zrcontent is set to 0.10% or less.

Cr: 0.02% or Less

If the Cr content is 0.02% or more, then there is a risk that thesurface after the anodization treatment will be tinged with a yellowcolor tone. Such a decrease in surface quality due to a change in colortone or the like can be prevented by restricting the Cr content to lessthan 0.02%.

Fe: 0.30% or Less, Si: 0.30% or Less, Mn: 0.02% or Less

Fe and Si are components that might be mixed into the aluminum ore asimpurities, and Mn is a component that might be mixed in if a recycledmaterial is used. Fe, Si, and Mn have the effect of inhibitingrecrystallization by forming AlMn-based, AlMnFe-based, or AlMnFeSi-basedintermetallic compounds with Al. Consequently, if the above-mentionedthree components are excessively mixed into the above-mentionedhigh-strength aluminum alloy material, then the formation of therecrystallized structure is inhibited and, instead, fibrous structurestend to be formed. If a fibrous structure is present, then after theanodization treatment is performed, a streak pattern caused by thefibrous structure tends to appear on the surface, and consequently thereis a risk of a reduction in surface quality. Such a reduction in surfacequality due to the streak pattern can be prevented by restricting Fe to0.30%% or less, Si to 0.30% or less, and Mn to 0.02% or less.

Ti: 0.001% or More and 0.05% or Less

By being added to the aluminum alloy, Ti has the effect of making theingot structure fine. The finer the ingot structure, the more a surfacestate of high luster without spots is obtained, and consequentlyincorporating Ti makes it possible to improve surface quality. If the Ticontent is less than 0.001%, then the ingot structure is not madesufficiently fine, and consequently there is a risk that spots andstreak patterns will arise on the surface of the above-mentionedhigh-strength aluminum alloy material. In addition, if the Ti content isgreater than 0.05%, then an AlTi-based intermetallic compound or thelike will be formed with the Al, and dot-like or streak pattern defectswill tend to be generated, and consequently there is a risk that surfacequality will decrease.

Next, as described above, the metallographic structure of theabove-mentioned high-strength aluminum alloy material comprises agranular recrystallized structure. Because an aluminum alloy prepared byperforming hot working normally has a metallographic structure composedof fibrous structures, there is a risk that streak patterns will ariseon the surface and, as a result, that the surface quality will decrease.On the other hand, in the above-mentioned high-strength aluminum alloy,the metallographic structure comprises a recrystallized structure, andconsequently streak patterns are not formed on the surface and thereforesurface quality is satisfactory.

In addition, in the above-mentioned high-strength aluminum alloy, a b*value (chromaticity of blue to yellow), which is part of the L*a*b*color system stipulated in JIS Z8729 (ISO 7724-1), measured after theanodization treatment using a sulfuric-acid bath is preferably 0 or moreand 0.8 or less. After the anodization treatment, an aluminum-alloymaterial having the b* value within the above-mentioned range has asuitable yellow-color density and becomes an aluminum-alloy materialthat excels in design characteristics.

By having at least the above-mentioned specific chemical composition,the above-mentioned high-strength aluminum alloy material can achieve acolor tone with a b* value of 0.8 or less. If the b* value exceeds 0.8,then the color tone after the anodization treatment will be tingedyellow, and consequently there is a risk of a deterioration in designcharacteristics. Furthermore, if the anodization treatment is performedon the aluminum-alloy material having the above-mentioned chemicalcomposition, then it is problematic to obtain the aluminum-alloymaterial having a b* value of less than 0.

In addition, in the above-mentioned recrystallized structure, theaverage grain diameter of the crystal grains is preferably 500 μm orless, and the crystal length in the direction parallel to thehot-working direction is preferably 0.5 times or more and 4 times orless than that of the crystal length in the direction perpendicular tothe hot-working direction.

If the above-mentioned average grain diameter of the crystal grainsexceeds 500 μm, then the crystal grains become excessively coarse, andconsequently spots tend to form on the surface after a surfacetreatment, such as the anodization treatment, is performed, andtherefore there is a risk that surface quality will decrease.Consequently, the smaller the average grain diameter of the crystalgrains, the better.

In addition, if the aspect ratio of the above-mentioned crystal grains,that is, the ratio of the crystal length in the direction parallel tothe hot-working direction with respect to the crystal length in thedirection perpendicular to the hot-working direction, exceeds 4, thenthere is a risk that streak patterns will appear on the surface afterthe anodization treatment has been performed. On the other hand, crystalgrains having an aspect ratio of less than 0.5 are difficult to obtainwith generally used manufacturing equipment.

Furthermore, it is possible to confirm whether the above-mentionedmetallographic structure is a recrystallized structure by, for example,electrolytic polishing the surface of the aluminum-alloy material andthen observing the resulting surface using a polarizing microscope. Thatis, if the above-mentioned metallographic structure is composed of arecrystallized structure, then a uniform metallographic structurecomposed of granular crystals will be observed, and a solidifiedstructure, which could be formed during casting, as represented bycoarse intermetallic compounds, floating crystals, and the like, willnot be seen. Similarly, a stripe-shaped structure (a so-called workedstructure) formed by plastic working, such as extrusion or rolling, willnot be seen in a metallographic structure composed of a recrystallizedstructure.

In addition, the average grain diameter of the crystal grains in theabove-mentioned recrystallized structure can be calculated bysectioning, in accordance with the sectioning method stipulated in JISG0551 (ASTM E 112-96, ASTM E 1382-97), the metallographic image obtainedby observation using the polarizing microscope described above. That is,the average grain diameter can be calculated by drawing, at an arbitraryposition in the above-mentioned metallographic image, onesectioning-plane line in each of the longitudinal, transverse, anddiagonal directions, and then dividing the length of eachsectioning-plane line by the number of crystal-grain boundaries thatintersect the sectioning-plane line.

In addition, the above-mentioned aspect ratio, that is, the ratio of thecrystal length in the direction parallel to the hot-working directionwith respect to the crystal length in the direction perpendicular to thehot-working direction, can be calculated in accordance with the methoddescribed above. That is, as in the method described above,sectioning-plane lines are drawn at an arbitrary position in theabove-mentioned metallographic image in the direction parallel to andthe direction perpendicular to the hot-working direction, and theaverage grain diameter is calculated in the direction parallel to andthe direction perpendicular to the hot-working direction from each ofthe sectioning-plane lines. Furthermore, the above-mentioned aspectratio can be calculated by dividing the average grain diameter in thedirection parallel to the hot-working direction by the average graindiameter in the direction perpendicular to the hot-working direction.

In addition, the above-mentioned recrystallized structure is preferablyone that is formed during hot working. The recrystallized structure canbe classified, depending on the manufacturing process, into a dynamicrecrystallized structure and a static recrystallized structure; arecrystallized structure that is formed through the performance ofrepetitive recrystallization simultaneous with deformation during thehot working is called a dynamic recrystallized structure. On the otherhand, a static recrystallized structure means one formed by firstperforming hot working or cold working, and then adding a heat-treatmentprocess, such as a solution heat treatment or an annealing treatment.The problem to be solved by the present invention described above can besolved for either recrystallized structure; however, in the case of thedynamic recrystallized structure, the production process is simple, andtherefore the structure can be manufactured easily.

Next, in the above-mentioned high-strength aluminum alloy materialmanufacturing method, a homogenization treatment is performed wherein aningot having the above-mentioned chemical composition is heated at atemperature above 540° C. and 580° C. or lower for 1 h or more and 24 hor less. If the heating temperature in the above-mentionedhomogenization treatment is 540° C. or lower, then the homogenization ofthe ingot segregation layer will be insufficient. As a result, thecrystal grains will become coarse, a nonuniform crystalline structurewill be formed, and the like, consequently reducing the surface qualityof the alloy material ultimately obtained. On the other hand, if theheating temperature is higher than 580° C., then there is a risk thatthe ingot will melt locally, consequently making manufacture difficult.Accordingly, the temperature of the above-mentioned homogenizationtreatment is preferably above 540° C. and 580° C. or lower.

In addition, if the heating time for the above-mentioned homogenizationtreatment is less than 1 h, then the homogenization of the ingotsegregation layer will be insufficient, and consequently the finalsurface quality will decrease in the same manner as described above. Onthe other hand, if the heating time exceeds 24 h, then a state willresult wherein the ingot segregation layer has been sufficientlyhomogenized, and consequently no further effect can be expected.Accordingly, the time for the above-mentioned homogenization treatmentis preferably 1 h or more and within 24 h.

The ingot subjected to the above-mentioned homogenization treatmentundergoes hot working and thereby is made into a wrought material. Thetemperature of the ingot at the start of the hot working is set to 440°C. or higher and 560° C. or lower. If the heating temperature of theingot before the hot working is lower than 440° C., then the deformationresistance will be high, making it difficult to work using generallyused manufacturing equipment. On the other hand, if the hot working isperformed after the ingot has been heated to a temperature that exceeds560° C., then the ingot locally melts owing to the inclusion of the heatgenerated during the working; as a result, there is a risk that hotcracking will occur. Accordingly, the temperature of the ingot beforethe hot working is preferably 440° C. or higher and 560° C. or lower.Furthermore, extruding, rolling, and the like can be employed as theabove-mentioned hot working.

In addition, after the above-mentioned hot working, a quenchingtreatment is performed wherein cooling is started while the temperatureof the wrought material is 400° C. or higher, and the temperature of thewrought material is then cooled until it becomes 150° C. or lower. Ifthe temperature of the wrought material before the above-mentionedquenching treatment is below 400° C., then the quench-hardening effectwill be insufficient and there is a risk that the proof stress of thewrought material obtained as a result will be less than 350 MPa. Inaddition, in the case wherein the temperature of the wrought materialafter the quenching treatment exceeds 150° C., too, the quench-hardeningeffect will be insufficient and there is a risk that the proof stress ofthe wrought material obtained as a result will be less than 350 MPa.

Furthermore, the above-mentioned quenching treatment means a treatmentthat cools the wrought material by a forcible means. For example,methods such as forcible quenching using a fan, shower cooling, watercooling, or the like can be employed as the above-mentioned quenchingtreatment.

In addition, in the above-mentioned quenching treatment, while thetemperature of the wrought material is in the range of from 400° C. to150° C., the average cooling rate is controlled such that it is 1° C./sor more and 300° C./s or less. If the average cooling rate exceeds 300°C./s, then excessively robust equipment will be needed and, moreover, acommensurate effect cannot be obtained. On the other hand, if theaverage cooling rate is less than 1° C./s, then the quench-hardeningeffect will be insufficient, and consequently there is a risk that theproof stress of the wrought material obtained will fall below 350 MPa.Accordingly, a faster average cooling rate is better, preferably 1° C./sor more and 300° C./s or less, and more preferably 3° C./s or more and300° C./s or less.

In addition, after the above-mentioned quenching treatment has beenperformed, the temperature of the wrought material is brought to roomtemperature. The temperature may be brought to room temperature by theabove-mentioned quenching treatment or by performing an additionalcooling treatment after the quenching treatment. Because the effect ofroom-temperature aging arises by virtue of bringing the temperature ofthe wrought material to room temperature, the strength of the wroughtmaterial increases. Furthermore, for example, methods such as fan aircooling, mist cooling, shower cooling, water cooling, or the like can beemployed as the above-mentioned additional cooling treatment.

Here, if the above-mentioned wrought material is stored in the statewherein it is maintained at room temperature, then the strength of thewrought material will further increase owing to the effect of theroom-temperature aging. In the initial stage, the longer theroom-temperature aging time, the greater the increase in strength;however, when the room-temperature aging time becomes 24 h or more, theeffect of room-temperature aging reaches its maximum.

Next, an artificial-aging treatment is performed wherein theabove-mentioned wrought material, which has been cooled to roomtemperature as described above, is heated. The performance of theartificial-aging treatment finely and uniformly precipitates MgZn₂ intothe above-mentioned wrought material, and consequently the proof stressof the wrought material can easily be set to 350 MPa or more. Any of theaspects below can be applied as specific conditions of theabove-mentioned artificial-aging treatment.

First, a first artificial-aging treatment, wherein the above-mentionedwrought material is heated at a temperature of 80-120° C. for 1-5 h, andthereafter a second artificial-aging treatment, which is performedfollowing the first artificial-aging treatment and wherein the wroughtmaterial is heated at a temperature of 145-200° C. for 2-15 h, can beperformed as the above-mentioned artificial-aging treatment.

Here, successively performing the first artificial-aging treatment andthe second artificial-aging treatment means completing the firstartificial-aging treatment and thereafter performing the secondartificial-aging treatment while maintaining the temperature of thewrought material. That is, the wrought material should not be cooledbetween the first artificial-aging treatment and the secondartificial-aging treatment; as a specific method, there is a methodwherein, after the first artificial-aging treatment, the secondartificial-aging treatment is performed without removing the wroughtmaterial from the heat-treatment furnace.

Thus, by successively performing the above-mentioned firstartificial-aging treatment and the above-mentioned secondartificial-aging treatment, the artificial-aging treatment time can beshortened. In addition, the treatment temperature of the secondartificial-aging treatment is preferably 145-200° C. If the heating inthe second artificial-aging treatment is performed in the range of170-200° C., then the ductility of the wrought material increases, andconsequently the workability can be further improved. Furthermore, ifthe conditions in the second artificial-aging treatment deviate from theabove-mentioned temperature range or time range, then there are risksthat stress corrosion cracking will tend to occur in the wroughtmaterial obtained, the proof stress will become less than 350 MPa, andthe like.

In addition, a treatment wherein the wrought material is heated at atemperature of 145-170° C. for 1-24 h can also be performed as theabove-mentioned artificial-aging treatment. In this case, themanufacturing process becomes simplified, and consequently manufacturecan be performed easily. If the above-mentioned artificial-agingtreatment deviates from the above-mentioned temperature range or timerange, then there is a risk that stress corrosion cracking will occur inthe wrought material obtained, the proof stress will become less than350 MPa, it will become difficult to obtain a wrought material havingsufficient strength properties, and the like.

WORKING EXAMPLES Working Example 1

Working examples according to the above-mentioned high-strength aluminumalloy material will be explained, making use of Table 1 to Table 3. Inthe present example, as shown in Table 1, samples (Nos. 1-30) havingvarying chemical compositions of the aluminum-alloy material wereprepared under the same manufacturing conditions, and the strength ofeach sample was measured and the metallographic structure of each samplewas observed. Furthermore, after each sample was subjected to a surfacetreatment, a surface-quality evaluation was performed.

Below, the manufacturing conditions, the strength measuring method, themetallographic structure observing method, as well as the surfacetreating method and the surface-quality-evaluating method of the sampleswill be explained.

TABLE 1 Sample Chemical Composition (mass %) Zn/Mg No. Zn Mg Cu Fe Si MnCr Zr Ti Al Ratio 1 5.2 2.4 0.01 0.11 0.09 0.01 0.01 0.01 0.018 bal. 2.12 6.8 2.4 0.06 0.12 0.08 0.01 0.01 0.00 0.021 bal. 2.9 3 5.9 2.2 0.010.12 0.09 0.01 0.01 0.00 0.017 bal. 2.7 4 5.9 2.8 0.08 0.12 0.08 0.010.01 0.00 0.007 bal. 2.1 5 5.2 2.9 0.04 0.11 0.09 0.01 0.01 0.00 0.018bal. 1.8 6 6.9 2.2 0.06 0.12 0.08 0.01 0.01 0.01 0.021 bal. 3.1 7 6.12.4 0.01 0.10 0.09 0.01 0.01 0.00 0.008 bal. 2.5 8 6.0 2.5 0.09 0.110.08 0.01 0.01 0.00 0.008 bal. 2.4 9 5.9 2.4 0.06 0.29 0.08 0.01 0.010.00 0.020 bal. 2.5 10 5.9 2.4 0.06 0.12 0.28 0.00 0.01 0.00 0.011 bal.2.5 11 5.9 2.5 0.06 0.13 0.09 0.02 0.00 0.00 0.009 bal. 2.4 12 5.8 2.50.05 0.12 0.08 0.01 0.02 0.03 0.013 bal. 2.3 13 5.8 2.5 0.05 0.12 0.020.01 0.00 0.05 0.012 bal. 2.3 14 5.8 2.5 0.05 0.12 0.08 0.01 0.01 0.000.002 bal. 2.3 15 5.8 2.5 0.05 0.12 0.09 0.00 0.01 0.00 0.043 bal. 2.3

TABLE 2 Sample Chemical Composition (mass %) Zn/Mg No. Zn Mg Cu Fe Si MnCr Zr Ti Al Ratio 16 4.9 2.3 0.08 0.12 0.09 0.01 0.01 0.01 0.012 bal.2.1 17 7.1 2.3 0.08 0.13 0.08 0.01 0.01 0.00 0.017 bal. 3.1 18 5.9 2.10.07 0.13 0.08 0.01 0.01 0.00 0.020 bal. 2.8 19 5.9 3.2 0.07 0.12 0.080.01 0.01 0.00 0.022 bal. 1.8 20 4.9 3.2 0.07 0.12 0.09 0.01 0.01 0.000.023 bal. 1.5 21 7.1 2.1 0.07 0.13 0.09 0.01 0.01 0.00 0.021 bal. 3.422 6.0 2.3 0.00 0.11 0.09 0.01 0.01 0.00 0.010 bal. 2.6 23 5.9 2.4 0.130.12 0.08 0.01 0.01 0.00 0.012 bal. 2.5 24 6.1 2.4 0.07 0.34 0.08 0.010.01 0.00 0.015 bal. 2.5 25 6.1 2.4 0.07 0.12 0.33 0.01 0.01 0.00 0.015bal. 2.5 26 6.1 2.4 0.07 0.13 0.09 0.04 0.00 0.00 0.017 bal. 2.5 27 6.12.4 0.07 0.12 0.09 0.01 0.04 0.00 0.016 bal. 2.5 28 6.1 2.4 0.06 0.120.09 0.01 0.01 0.07 0.018 bal. 2.5 29 5.9 2.4 0.06 0.12 0.08 0.01 0.010.00 0.000 bal. 2.5 30 6.0 2.4 0.06 0.07 0.08 0.01 0.01 0.01 0.070 bal.2.5

<Manufacturing Conditions of Samples>

Ingots having a diameter of 90 mm and the chemical compositions listedin Table 1 and Table 2 were cast by semi-continuous casting.Subsequently, a homogenization treatment was performed wherein theingots were heated at a temperature of 550° C. for 6 h. Subsequently,the ingots were hot extruded in the state wherein the temperature of theingots was 520° C., thereby forming wrought materials having a width of35 mm and a thickness of 7 mm. Subsequently, a quenching treatment wasperformed in which, in the state wherein the temperature of the wroughtmaterials was 505° C., the wrought materials were cooled to 100° C. atan average cooling rate of 60° C./s. Furthermore, the wrought materialssubjected to the above-mentioned quenching treatment were cooled to roomtemperature and then subjected to room-temperature aging for 24 h atroom temperature. Subsequently, the first artificial-aging treatment wasperformed wherein the above-mentioned wrought materials were heatedusing a heat-treatment furnace at a temperature of 100° C. for 4 h.Thereafter, the second artificial-aging treatment was performed whereinthe furnace temperature was raised to 160° C. without removing theheating wrought materials from the heat-treatment furnace, and thewrought materials were heated at 160° C. for 8 h, thereby making thesamples.

<Strength Measuring Method>

Test pieces were collected from the samples using the method inaccordance with JIS Z2241 (ISO 6892-1), and tension tests that measuretensile strength, proof stress, and elongation were performed. Thestrength characteristic of those exhibiting a proof stress of 350 MPa ormore in the tension test results was judged to be acceptable.

<Metallographic Structure Observing Method>

After the samples were subjected to electrolytic polishing andelectrolytic etching, micrographs of the sample surfaces were acquiredusing a polarizing microscope having a magnification of 50-100 times.Image analysis was performed on the micrographs and, as described above,the average grain diameter of the crystal grains constituting themetallographic structure of each of the samples was derived inaccordance with the sectioning method stipulated in JIS G0551. Inaddition, as described above, each of the aspect ratios (indicating theratio of the crystal length in the direction parallel to the hot-workingdirection with respect to the crystal length in the directionperpendicular to the hot-working direction) was calculated by dividingthe average grain diameter in the direction parallel to the hot-workingdirection by the average grain diameter in the direction perpendicularto the hot-working direction. As a result, those having an average graindiameter of 500 μm or less and those having an aspect ratio within arange of 0.5-4.0 were judged to be preferable results.

<Surface Treating Method>

After buffing the surfaces of the samples that were subjected to theartificial-aging treatment, the samples were etched with an aqueoussolution of sodium hydroxide and afterward subjected to a desmuttingtreatment. The samples subjected to the desmutting treatment werechemically polished using a phosphoric acid—nitric acid method at atemperature of 90° C. for 2 min. Furthermore, the samples subjected tothe chemical polishing were subjected to an anodization treatment at anelectric current density of 150 A/m² in a 15% sulfuric-acid bath,thereby forming 10-μm anodic oxide films. Lastly, the anodic oxide filmswere subjected to a sealing treatment by immersing the samples, afterthey were subjected to the anodization treatment, in boiling water.

<Surface-Quality-Evaluating Method>

The surfaces of the samples subjected to the above-mentioned surfacetreatment were visually observed. In the visual observation, thosewherein a streak pattern, a spotting pattern, a dot-like defect, or thelike did not appear on the surface were judged to be acceptable.

Subsequently, the color tone of the surfaces of the samples weremeasured using a color-difference meter to obtain the coordinate valuesin the L*a*b* color system described in JIS Z8729. As a result, thosehaving a b* value (chromaticity of blue to yellow) within a range of0-0.8 were judged to be acceptable.

<Stress-Corrosion-Cracking Testing Method>

Tests were performed in accordance with JIS H8711 (ISO 9591). A C-ringshaped test piece was cut out from each sample, the test piece beingprovided with a notched part in a portion along the circumference of thering shape having an outer diameter of 20 mm, an inner diameter of 17mm, and an axial-direction thickness of 7 mm. The direction thatconnects the center of the C-ring shape with the notched part is alignedwith the extrusion direction during sample preparation. In stressloading the test piece, a stress of 330 MPa is loaded in the directionthat compresses the C-ring shape in the direction orthogonal to theabove-mentioned extrusion direction. In this loaded state, alternatingimmersion is performed for 720 h in an atmosphere having a temperatureof 25° C., during which each test piece is alternately immersed in a3.5% aqueous solution of NaCl for 10 min and then dried for 50 min. Thetest results were judged based on the presence or absence of crackgeneration. Those without cracks were assigned “good” (∘), and thosewith cracks were assigned “bad” (x).

The evaluation results of the samples listed in Table 1 and Table 2 areshown in Table 3. Furthermore, those evaluation results in Table 3 notjudged to be acceptable or not judged to be a preferable result areunderlined.

TABLE 3 Average Stress- Tensile Crystal Grain Presence Corrosion- SampleStrength Proof Stress Elongation Diameter Aspect of Streak b* CrackingNo. (MPa) (Mpa) (%) (μm) Ratio Pattern Value Resistance 1 396 367 21 1201.8 No 0.3 ∘ 2 468 448 14 110 1.5 No 0.2 ∘ 3 388 355 18 100 1.2 No 0.3 ∘4 475 460 16 100 1.2 No 0.3 ∘ 5 390 435 21 100 1.3 No 0.4 ∘ 6 461 438 16100 1.3 No 0.4 ∘ 7 455 425 17 110 1.4 No 0.4 ∘ 8 460 431 18 110 1.2 No0.6 ∘ 9 467 438 14 120 1.2 No 0.3 ∘ 10 464 435 15 120 1.2 No 0.2 ∘ 11461 432 15 130 1.2 No 0.3 ∘ 12 466 438 16 100 1.4 No 0.2 ∘ 13 465 435 16120 1.4 No 0.7 ∘ 14 463 431 16 120 3.5 No 0.2 ∘ 15 468 438 16 130 1.3 No0.3 ∘ 16 379 348 22 110 1.2 No 0.3 ∘ 17 471 452 13 110 1.3 No 0.3 x 18382 347 21 110 1.2 No 0.2 ∘ 19 481 450 12 120 1.5 No 0.2 x 20 379 349 21110 1.3 No 0.3 ∘ 21 462 431 15 110 1.2 No 0.3 x 22 468 438 16 120 1.3 No0.4 x 23 467 438 16 100 1.2 No 1.1 ∘ 24 466 435 16 — >4   Yes 0.3 ∘ 25467 437 16 — >4   Yes 0.4 ∘ 26 469 438 15 — 1.3 Yes 0.4 ∘ 27 462 433 16150 1.5 Yes 1.3 ∘ 28 467 437 16 — >4   Yes 0.3 ∘ 29 467 438 15 >500  1.8Yes 0.4 ∘ 30 464 435 15 100 1.5 Yes 0.3 ∘

As can be understood from Table 3, samples 1-15 were acceptable for allevaluation items and exhibited excellent characteristics for strength,surface quality, and stress-corrosion-cracking resistance.

As a representative example of a sample having excellent surfacequality, FIG. 1 shows the metallographic structure observation result ofsample 4. As can be understood from the same figure, samples having anexcellent surface quality have a metallographic structure composed of agranular recrystallized structure and, simultaneously, no streak patternis observed even by visual confirmation and the samples have a highluster without any spots.

In sample 16, the Zn content was too low, and consequently a sufficientstrength improving effect was not obtained and therefore the proofstress was judged to be unacceptable.

In sample 17, the Zn content was too high, and consequentlystress-corrosion-cracking resistance was poor and was judged to beunacceptable.

In sample 18, the Mg content was too low, and consequently a sufficientstrength improving effect was not obtained and the proof stress wasjudged to be unacceptable.

In sample 19, the Mg content was too high, and consequently cracksformed in portions during extrusion; furthermore,stress-corrosion-cracking resistance was poor and was judged to beunacceptable.

In sample 20, the Zn/Mg ratio was too low, and consequently strength waspoor and was judged to be unacceptable.

In sample 21, the Zn/Mg ratio was too high, and consequentlystress-corrosion-cracking decreased and was judged to be unacceptable.

In sample 22, the Cu content was too low, and consequentlystress-corrosion-cracking resistance was poor and judged to beunacceptable.

In sample 23, the Cu content was too high, and consequently the surfacecolor tone was tinged yellow and judged to be unacceptable.

In sample 24, the Fe content was too high, and consequently a fibrousstructure was formed; as a result, a streak pattern was visuallyconfirmed on the surface and judged to be unacceptable.

In sample 25, the Si content was too high, and consequently a fibrousstructure was formed; as a result, a streak pattern was visuallyconfirmed on the surface and judged to be unacceptable.

In sample 26, the Mn content was too high, and consequently a fibrousstructure was formed; as a result, a streak pattern was visuallyconfirmed on the surface and judged to be unacceptable.

In sample 27, the Cr content was too high, and consequently the surfacecolor tone was tinged with yellow and judged to be unacceptable.

In sample 28, the Zr content was too high, and consequently a fibrousstructure was formed; as a result, a streak pattern was visuallyconfirmed on the surface and judged to be unacceptable.

In sample 29, the Ti content was too low, and consequently a streakpattern caused by the coarse ingot structure appeared and was judged tobe unacceptable.

In sample 30, the Ti content was too high, and consequently the Tiformed an intermetallic compound with the Al; as a result, stripe shapesand dot-like defects were visually confirmed on the surface and judgedto be unacceptable.

Working Example 2

Next, a working example according to the above-mentioned high-strengthaluminum alloy manufacturing method will be explained, making use ofTable 4 to Table 6.

In the present example, samples (No. A1-A29) were prepared, using analuminum alloy (material No. A) containing the chemical compositionlisted in Table 4, under varying manufacturing conditions as listed inTable 5 and Table 6, after which the strength of each sample wasmeasured and the metallographic structure of each sample was observed.Furthermore, after each sample was subject to a surface treatment, asurface-quality evaluation was performed.

Below, the manufacturing conditions of each sample will be explained.Furthermore, the strength measuring method, the metallographic structureobserving method, the surface treating method, and thesurface-quality-evaluating method for each sample were performed usingthe same methods as those in Working Example 1.

<Manufacturing Conditions of Samples>

An ingot having a diameter of 90 mm and the chemical composition listedin Table 4 was cast by semi-continuous casting. Subsequently, usingcombinations of the temperatures, times, and average cooling rateslisted in Table 5 and Table 6, the above-mentioned ingot was subjectedto, in order, a homogenization treatment, hot extrusion, a quenchingtreatment, a first artificial-aging treatment, and a secondartificial-aging treatment, and thereby the samples were obtained.Furthermore, “room-temperature aging time” in Table 5 and Table 6 meansthe time from when the wrought material reaches room temperature afterthe performance of the quenching treatment until the performance of thefirst artificial-aging treatment.

TABLE 4 Sample Chemical Composition (mass %) Zn/Mg No. Zn Mg Cu Fe Si MnCr Zr Ti Al Ratio A 6.0 2.3 0.04 0.11 0.06 0.00 0.01 0.00 0.012 bal. 2.6

TABLE 5 Quenching Room- Homogenization Hot Working Treatment Temp. FirstArtificial Second Artifical Treatment Extruding Cooling Final AgingAging Aging Sample Temp. Time Temp. Rate Temp. Time Temp. Time Temp.Time No. (° C.) (h) (° C.) (° C./s) (° C.) (h) (° C.) (h) (° C.) (h) A1540 6 520 60 100 24 100 4 160 8 A2 577 6 520 60 100 24 100 4 160 8 A3550 1 520 60 100 24 100 4 160 8 A4 550 24 520 60 100 24 100 4 160 8 A5550 6 440 60 100 24 100 4 160 8 A6 550 6 560 60 100 24 100 4 160 8 A7550 6 520 1 100 24 100 4 160 8 A8 550 6 520 300 100 24 100 4 160 8 A9550 6 520 60 150 24 100 4 160 8 A10 550 6 520 60 100 None 100 4 160 8A11 550 6 520 60 100 240 100 4 160 8 A12 550 6 520 60 100 24 80 5 160 8A13 550 6 520 60 100 24 120 1 160 8 A14 550 6 520 60 100 24 100 4 145 15A15 550 6 520 60 100 24 100 4 200 2 A16 550 6 520 60 100 24 145 15 — —A17 550 6 520 60 100 24 170 2 — —

TABLE 6 Quenching Room- Homogenization Hot Working Treatment Temp. FirstArtificial Second Artifical Treatment Extruding Cooling Final AgingAging Aging Sample Temp. Time Temp. Rate Temp. Time Temp. Time Temp.Time No. (° C.) (h) (° C.) (° C./s) (° C.) (h) (° C.) (h) (° C.) (h) A18535 6 520 60 100 24 100 4 160 8 A19 550   0.25 520 60 100 24 100 4 160 8A20 550 6 570 — — — — — — — A21 550 6 520    0.25 100 24 100 4 160 8 A22550 6 520 60 100 24  80 3 140 14  A23 550 6 520 60 100 24 120 5 210 2A24 550 6 520 60 100 24 100 4 145 1 A25 550 6 520 60 100 24 100 4 20016  A26 550 6 520 60 100 24 140 23  — — A27 550 6 520 60 100 24 180 2 —— A28 550 6 520 60 100 24 165   0.25 — — A29 550 6 520 60 100 24 165 30 — —

The evaluation results of the samples prepared as described above arelisted in Table 7. Furthermore, those measurement results in Table 7that were not judged to be acceptable or were not judged to have apreferable result are underlined.

TABLE 7 Average Stress- Tensile Crystal Grain Presence Corrosion- SampleStrength Proof Stress Elongation Diameter Aspect of Streak b* CrackingNo. (MPa) (Mpa) (%) (μm) Ratio Pattern Value Resistance A1 398 366 21130 1.2 No 0.3 ∘ A2 488 479 14 110 1.3 No 0.3 ∘ A3 386 355 20 120 1.2 No0.3 ∘ A4 489 468 13 100 1.1 No 0.2 ∘ A5 398 368 22 120 1.3 No 0.3 ∘ A6486 471 14 120 1.2 No 0.3 ∘ A7 389 358 19 120 1.2 No 0.2 ∘ A8 486 472 14130 1.3 No 0.3 ∘ A9 393 363 19 120 1.2 No 0.3 ∘ A10 484 462 14 110 1.2No 0.2 ∘ A11 483 463 14 110 1.2 No 0.3 ∘ A12 399 369 21 110 1.2 No 0.2 ∘A13 451 421 15 120 1.2 No 0.3 ∘ A14 482 461 12 120 1.2 No 0.2 ∘ A15 391362 21 120 1.3 No 0.3 ∘ A16 479 458 13 120 1.1 No 0.2 ∘ A17 392 363 21110 1.1 No 0.4 ∘ A18 377 349 22 210 >4   Yes 0.3 ∘ A19 376 346 20 5101.3 Yes 0.3 ∘ A20 — — — — — — — — A21 360 330 22 120 1.1 No 0.3 ∘ A22480 452 14 120 1.1 No 0.2 x A23 372 340 21 140 1.2 No 0.3 ∘ A24 375 34520 130 1.2 No 0.2 x A25 369 341 20 120 1.3 No 0.3 ∘ A26 479 451 15 1201.4 No 0.2 x A27 370 341 21 120 1.2 No 0.3 ∘ A28 371 342 21 110 1.2 No0.2 ∘ A29 360 349 20 120 1.3 No 0.3 ∘

As can be understood from Table 7, samples A1-A17 were acceptable forall evaluation items and exhibited characteristics excelling in bothstrength and surface quality.

In sample A18, the heating temperature in the homogenization treatmentwas too low, and consequently the proof stress was less than 350 MPa andjudged to be unacceptable. Simultaneously, the crystal grains becamecoarse and a spotting pattern was also visually confirmed on thesurface.

In sample A19, the treatment time of the homogenization treatment wastoo short, and consequently the proof stress was less than 350 MPa andjudged to be unacceptable. Simultaneously, the crystal grains becamecoarse and a spotting pattern was also visually confirmed on thesurface.

In sample A20, the heating temperature of the ingot, before the hotextrusion, was too high, and consequently the ingot partially meltedduring the extrusion; as a result, hot-working cracks formed andtherefore treatments subsequent to the quenching treatment could not beperformed.

In sample A21, the average cooling rate of the quenching treatment wastoo low, and consequently the quench-hardening effect was insufficientand the proof stress was less than 350 MPa and judged to beunacceptable.

In sample A22, the treatment temperature of the second artificial-agingtreatment was too low, and consequently stress-corrosion-crackingresistance was insufficient and judged to be unacceptable.

In sample A23, the treatment temperature of the second artificial-agingtreatment was too high, and consequently overaging occurred and theproof stress was less than 350 MPa and judged to be unacceptable.

In sample A24, the treatment time of the second artificial-agingtreatment was too short, and consequently the age hardening wasinsufficient, the proof stress was less than 350 MPa, andstress-corrosion-cracking resistance was also insufficient and judged tobe unacceptable.

In sample A25, the treatment time of the second artificial-agingtreatment was too long, and consequently overaging occurred and theproof stress was less than 350 MPa and judged to be unacceptable.

In sample A26, only one stage of artificial-aging treatment wasperformed, and the treatment temperature of that artificial-agingtreatment was too low, and consequently stress-corrosion-crackingresistance was insufficient and judged to be unacceptable.

In sample A27, only one stage of artificial-aging treatment wasperformed, and the treatment temperature of that artificial-agingtreatment was too high, and consequently overaging occurred and theproof stress was less than 350 MPa and judged to be unacceptable.

In sample A28, the treatment time of the first artificial-agingtreatment was too short, and consequently the age hardening wasinsufficient and the proof stress was less than 350 MPa and judged to beunacceptable.

In sample A29, the treatment time of the first artificial-agingtreatment was too long, and consequently overaging occurred and theproof stress was less than 350 MPa and judged to be unacceptable.

FIG. 2 shows the result of the observation of the metallographicstructure of sample A19 as a representative example of a sample, fromamong the samples where the surface quality was unacceptable, in which astreak pattern was visually confirmed. As can be understood from thesame figure, samples wherein streak patterns were visually confirmedhave a metallographic structure composed of fibrous structures.

1.-5. (canceled)
 6. An aluminum alloy, comprising in mass percent: Zn:5.0% or more and 7.0% or less, Mg: more than 2.2% and 3.0% or less, Cu:0.01% or more and 0.10% or less, Zr: 0.10% or less, Cr: 0.02% or less,Fe: 0.30% or less, Si: 0.30% or less, Mn: 0.02% or less, and Ti: 0.001%or more and 0.05% or less, the remainder being composed of Al andunavoidable impurities, wherein the aluminum alloy has: a Zn/Mg ratio of1.7 or more and 3.1 or less, a proof stress of 350 MPa or more, and ametallographic structure composed of a recrystallized structure.
 7. Thealuminum alloy according to claim 6, wherein: the recrystallizedstructure includes crystal grains having an average grain diameter of500 μm or less, and a crystal grain length in a direction parallel to ahot working direction is 0.5 to 4 times as long as a crystal grainlength in a direction perpendicular to the hot working direction.
 8. Thealuminum alloy according to claim 7, wherein Zn is more than 5.2% and6.8% or less.
 9. The aluminum alloy according to claim 8, wherein thealuminum alloy has an anodized surface that has a b* value of 0 or moreand 0.8 or less.
 10. The aluminum alloy according to claim 9, whereinthe recrystallized structure is a granular recrystallized structure. 11.The aluminum alloy according to claim 6, wherein Zn is more than 5.2%and 6.8% or less.
 12. The aluminum alloy according to claim 6, whereinthe aluminum alloy has an anodized surface that has a b* value of 0 ormore and 0.8 or less.
 13. The aluminum alloy according to claim 6,wherein the recrystallized structure is a granular recrystallizedstructure.
 14. A process for producing a wrought aluminum alloymaterial, which comprises: preparing an ingot having a chemicalcomposition comprising, in mass percent, Zn: 5.0% or more and 7.0% orless, Mg: more than 2.2% and 3.0% or less, Cu: 0.01% or more and 0.10%or less, Zr: 0.10% or less, Cr: 0.02% or less, Fe: 0.30% or less, Si:0.30% or less, Mn: 0.02% or less, and Ti: 0.001% or more and 0.05% orless, the remainder being composed of Al and unavoidable impurities anda Zn/Mg ratio being 1.7 or more and 3.1 or less; performing ahomogenization treatment that heats the ingot at a temperature of higherthan 540° C. and 580° C. or lower for 1 hour to 24 hours; subsequently,forming a wrought material by performing hot working on the ingot in astate where the temperature of the ingot at the beginning of the workingis 440° C. to 560° C.; while the wrought material is still at 400° C. orhigher, starting to cool it and subsequently performing a quenchingtreatment such that, while the wrought material is cooling down from400° C. to 150° C., the average cooling rate is 1° C./s or more and 300°C./s or less; cooling the temperature of the wrought material to roomtemperature by said quenching treatment or by an additional coolingtreatment; and thereafter, performing an artificial aging treatment onthe wrought material.
 15. The process according to claim 14, wherein theartificial aging treatment comprises performing a first artificial agingtreatment at a temperature of 80° C. to 120° C. for 1 hour to 5 hours,and continuously after the first artificial aging treatment, performinga second artificial aging treatment that heats the wrought material at atemperature of 145° C. to 200° C. for 2 hours to 15 hours.
 16. Theprocess according to claim 14, wherein the artificial aging treatmentcomprises heating the wrought material at a temperature of 145° C. to170° C. for 1 hour to 24 hours.
 17. The process according to claim 14,wherein the hot working involves extrusion or rolling.
 18. The processaccording to claim 14, wherein during the quenching step, the averagecooling rate is 3° C./s or more and 300° C./s or less
 19. The processaccording to claim 14, wherein the second artificial aging treatment isperformed at a temperature of 170° C. to 200° C.
 20. The processaccording to claim 14, further comprising anodizing the wrought materialafter the artificial aging treatment.
 21. The process according to claim14, wherein: the homogenization treatment is performed at 550° C. for 6hours, the hot working comprises hot extruding the ingot while thetemperature of the ingot is at 520° C., the quenching treatment isinitiated while the temperature of the wrought material is at 505° C.and the average cooling rate of the quenching treatment is 60° C./sec,thereafter the wrought material is subjected to room temperature agingfor 24 hours, the first artificial aging treatment involves heating thewrought material at 100° C. for 4 hours, and the second artificial agingtreatment involves heating the wrought material at 160° C. for 8 hours.22. The process according to claim 21, further comprising anodizing thewrought material after the second artificial aging treatment.
 23. Aprocess for producing the aluminum alloy of claim 6, comprising:homogenizing an ingot having the elemental composition recited in claim6 at a temperature of higher than 540° C. and 580° C. or lower for atleast 1 hour; hot working on the homogenized ingot while the temperatureof the ingot at the beginning of the hot working is 440° C. to 560° C.,thereby forming a wrought material, while the wrought material is stillat 400° C. or higher, starting to cool it and subsequently performing aquenching treatment such that, while the wrought material is coolingdown from 400° C. to 150° C., the average cooling rate is 1° C./s ormore and 300° C./s or less; cooling the temperature of the wroughtmaterial to room temperature by said quenching treatment or by anadditional cooling treatment; and thereafter, performing an artificialaging treatment on the wrought material.
 24. The process according toclaim 23, wherein the artificial aging treatment comprises performing afirst artificial aging treatment at a temperature of 80° C. to 120° C.for 1 hour to 5 hours, and continuously after the first artificial agingtreatment, performing a second artificial aging treatment that heats thewrought material at a temperature of 145° C. to 200° C. for 2 hours to15 hours.
 25. The process according to claim 23, wherein the artificialaging treatment comprises heating the wrought material at a temperatureof 145° C. to 170° C. for 1 hour to 24 hours.